循环26~28

循环26

#include<iostream>

#include<iomanip>

int main()

{
using namespace std;
cout << "Enter an integer :";
int n; cin >> n;
cout << fixed << setprecision(3);
double sum=0.0;
for (int i = 1; i < n + 1; i++)
sum = sum + 1.0 / double(3 * i - 2)*pow(-1,i-1);
cout << "sum = " << sum;
return 0;

}

循环27

#include<iostream>

#include<iomanip>

int main()

{
using namespace std;
cout << "Enter an integer :";
int n; cin >> n;
cout << fixed << setprecision(6);
double sum = 0;
for (int i = 1; i < n + 1; i++)
sum = sum + 1.0 / double(i);
cout << "sum = " << sum;
return 0;

}

循环28

#include<iostream>

int main()

{
using namespace std;
long sum = 0;
cout << "Enter an integer :";
int n; cin >> n;
if (n < 2)
{
cout << "Invalid input !\n"; return 0;
}
for (int i = 1; i < n + 1; i++)
sum = sum + pow(2, i);
cout << "sum = " << sum;
return 0;

}